At present, TN (twisted nematic) type display mode has been most broadly used. This TN display has many advantages such as low driving voltage, low power consumption, etc. However, it is far inferior in the aspect of response speed to emissive display elements such as cathode ray tube, electroluminescence, plasma display, etc. A new type TN display element having a twist angle increased up to 180.degree. to 270.degree. has also been developed, but it is still yet inferior in the aspect of response speed. Efforts for various improvements have been made as described above, and nevertheless, development of TN display element having a high response speed has not yet been realized.
However, as to a new display mode using a ferroelectric liquid crystal which has now been extensively researched, a notable improvement in the response speed has been anticipated (Clark et al, Applied Phys. lett., 36, 899 (1980)). This display mode utilizes chiral smectic phases exhibiting ferroelectricity such as chiral smectic C phase (hereinafter abbreviated to Sc* phase), etc.
A number of specific features have been required for ferroelectric liquid crystal materials having been used for ferroelectric liquid crystal display elements practically used. Representative items of the above specific features are spontaneous polarization (Ps), tilt angle (.theta.), viscosity (.eta.), liquid crystal phase sequence, etc.
The molecule in the ferroelectric liquid crystals can move only on a cone, and it can form two states wherein the parallel direction of the molecule is perpendicular to the electric field corresponding to a direction of the electric field. The angle made between the two states is referred as cone angle and the half of the cone angle is referred to as tilt angle (.theta.).
The ferroelectric liquid crystal display mode currently includes mainly two modes i.e. a mode referred to as a birefringence mode wherein two upper and lower polarizers are used, and a mode referred to as guest-host mode (G.H.) wherein one polarizer is used and a dichroic dyestuff is added. In order that the ratio of brightness of the bright and dark states (contrast ratio) is the best, a tilt angle of 22.5.degree. is required for the birefringence mode, while a tilt angle of 45.degree. is required for the G.H. mode.
Further, since a relationship of .tau..infin..eta./Ps is existent among the responce time (.tau.), Ps and .eta., a material having a larger Ps and a lower .eta. is required for making the response speed higher. A number of specific features are required for ferroelectric liquid crystal materials used for ferroelectric liquid crystal display elements practically used, as described above, but any single compound cannot satisfy the requirements in the current state. Hence the ferroelectric liquid crystal compositions have been provided in the form of a mixture of a numer of materials. The compositions may be composed of even either liquid crystal compounds or non-liquid crystalline compounds. Ferroelectric liquid crystal compositions can be prepared by composing only of ferroelectric liquid crystal compounds, or by a method of mixing as a basic substance, a compound or a composition exhibiting tilted smectic phases of achiral smectic C, F, G, H, I, etc. (hereinafter abbreviated to Sc phase etc.), with one member or more of ferroelectric liquid crystal compounds or non-liquid crystalline, optically active compounds to thereby form a composition exhibiting a ferroelectric liquid crystalline phase as a whole.
As such a basic substance, a group of compounds of various series exhibiting achiral smectic liquid crystal phases of Sc etc. have been used, but liquid crystal compounds or liquid crystal compositions exhibiting smectic phases within a broad temperature range of from low temperatures to room temperature or higher have been practically used. Sc phases among the smectic phases exhibit the highest speed response properties among the ferroelectric liquid crystal phases, hence the Sc phases broadly and generally constitute the liquid crystal phases of the basic substance. As the constituting components of these smectic C liquid crystal compositions, liquid crystal compounds such as those of phenyl benzoate group, Schiff's base group, biphenyl group, phenylpyridine group, phenylpyrimidine group, etc. are exemplified. Representative examples of these compounds are as follows wherein R.sup.8 and R.sup.9 each independently represent an alkyl group or alkoxy group of 1 to 20 carbon atoms: ##STR2##
Further, as compounds to be added to such basic substances and inducing ferroelectricity, many compounds have been reported so far.
Examples of compounds having been used as such optically active raw materials are as follows:
2-methylbutanol, 2-octanol, lactic acid esters, 1-trifluoromethyl-1-heptanol, ethyl 4,4,4-trifluoro-3-hydroxybutanoate, amino acids such as isoleucine, epoxy octane, etc.
Further, a number of compounds for ferroelectric liquid crystal compositions using lactic acid as an optically active raw material have been tried. These compounds are mentioned below: ##STR3##
However, the ferroelectric liquid crystal display element has not yet been practically used, since the specific features required for the practical element could not have been satisfied by currently known compounds. The main cause consists in that optically active raw materials having been used so far have been limited.
The object of the present invention is to provide a novel optically active liquid crystal material by using a novel optically active raw material.